The candidate is in a unique position to integrate two broad areas of knowledge, having been trained in both molecular immunology, and clinical pathology. As previous Ph.D. work involved the analysis of the molecular function of CD8 in vitro, the candidate requires this additional training period to learn to apply basic research skills to the study of aberrant cell growth in whole animals. This will provide a solid basis for a career dedicated to elucidating molecular mechanisms that contribute to the development of human hematopoietic neoplasms. The mechanisms that control the differentiation of cells from one developmental stage to the next are poorly understood. Not only may a single gene product regulate several transition points, but it may function by distinct mechanisms at different times. While highly complex, a whole animal model is the most appropriate system in which to identify and analyze the various roles which a regulatory protein may play during development. The understanding of hematopoietic development has been greatly advanced by the germline introduction of transgenes and gene disruption experiments in mice. One molecule likely to play a key role in hematopoietic development is p50csk, a putative negative regulator of src-family protein tyrosine kinases (PTKs). However, as for many other important regulatory proteins, the study of p50csk in vivo is problematic because csk gene disruption gives an embryonic lethal phenotype. The broad objective of this proposal is to develop a whole animal model for the regulated disruption of genes that control thymocyte growth and differentiation. Using the bacteriophage P1 Cre recombinase system a mouse model will be created in which the deletion of the endogenous csk gene can be achieved at defined points of thymocyte maturation. By the alteration of oncogenes in somatic cells, this will also serve as a powerful model of human malignancy. Specific aims are: (i) To express Cre under the control of several T-cell specific promoter elements, and achieve csk gene disruption in transgenic mice at specific stages of thymocyte development. (ii) To analyze the phenotype of thymocytes which show developmental arrest or malignant transformation secondary to csk loss. (iii) To test the hypothesis that loss of csk during thymocyte development will lead to dysregulation of the T-cell specific PTKs p56lck and p59fyn(t). The sponsor's lab provides an ideal environment in which to pursue the proposed experiments since mouse embryo manipulation has been successfully used by this group to study T- cell development for more than six years. All of the techniques required for success of the proposed work have already been implemented there.